How should my characters fall in love?

I'm finding it hard to incorporate romance into my story. The MC is a boy called Luca who falls in love with a girl called Reese after he follows her into another world because he suspects she attacked his parents. How should they fall in love? They used to know each other from school so it can't be love at first sight for him... Should they realize it eventually or should she love him since she love him since she met him? Thanks :-)